The present invention relates to a process for manufacturing heat exchangers from ceramic sheets in which sheets are formed, stacked, laminated and dried, together with an apparatus for producing such heat exchangers from individual sheets.
German Published Application No. DE-OS 28 41 571 discloses a process for producing heat exchangers from ceramic sheets in which stamped sheets with spacers therebetween are stacked between two base plates and so-called windows are additionally machined in the covering walls. The resulting block-shaped heat exchangers are subsequently subjected to a cold or hot laminating process. The production costs of such a process are higher than the costs of producing conventional extruded ceramic heat exchangers, but very thin walls are obtained. Furthermore, the extrusion method does not permit installation of so-called baffles transversely to the direction of drawing of the flow channels. Also, handling during assembly of heat exchangers from rods and thin sheets is very difficult, and the production method is highly labor intensive. Further, it has been found during the laminating that the sheets do not all adhere uniformly to each other and particularly that the forming tools are easily clogged or fouled during the green processing of unsintered heat exchanger blocks due to the organic binder content of the sheets. If all of the binder is removed from the ceramic, the body becomes very brittle so that processing again becomes difficult.
United Kingdom Pat. No. 1,418,459 discloses a process for manufacturing heat exchangers from sheets. Sheets having a thickness of approximately 0.15 mm are produced on a combustible carrier material by a doctor blade. It has thereby been found to be especially disadvantageous that the spacers between the separating walls for the heat exchange media are made by a very expensive technique poorly suited for mass production. The heat exchanger is constructed by alternately stacking silicon synthetic resin sheets and spacers attached to cast sheets. Using pressure and heat as well as a solvent or an adhesive, the individual parts of the heat exchanger are assembled. During firing, first the paper must be removed, then the binder and finally, the nitriding process is effected. During combustion of the paper, care must be taken not to damage the fine silicon structure. The ash formed in the process is removed by ultrasonic cleaning. Furthermore, prior to burning the paper, partial nitriding of the heat exchanger block must be effected.
The shortcomings found in the described processes do not permit rational mass production. Additionally, the completed heat exchangers often exhibit non-homogeneous structures. In particular, it has been found that the flow behavior of heat exchangers made of silicon nitride is not optimal, since as a result of the porous surface of silicon nitride, smooth flow channels are not obtained.